1. Field of the invention
The present invention concerns optical memory devices providing access to areas reserved for storage of data and defined by appropriately arranging the reference surface of a disk.
2. Description of the prior art
The data is stored by means of a read-write device that essentially comprises drive means for rotating the disk and a radially movable optical head which projects onto the reference surface through an objective lens a finely focussed light spot. The disk is prewritten to show up in the reference surface the areas intended to receive the data. These areas are situated on a spiral track or on concentric circular track elements. The essential tasks for the optical head are to read the prewritten information, to write data, to read data and, where appropriate, to erase data. To implement these various tasks when the data on the disk is highly dense it is necessary to provide focussing and track following control systems. Scanning the track also makes it necessary to provide synchronization means, because each element of data is assigned to elementary sites disposed serially along the track to which reference is made by means of synchronization patterns situated on specific radii that are equi-angularly distributed.
Given that the disk comprises a structure that is optically writable by reversibly or irreversibly modifying its optical characteristics, it is prudent to avoid writing data in certain parts of the reference surface that are periodically scanned to implement the synchronization, track following and focussing functions. The areas reserved for writing data are then free of any prewritten information or, in the limit, provided with only a prewritten groove which plays no role in controlling the light beam projected by the optical head. A disk that has been preformatted according to this principle is generally read by a write-read device which responds to samples in the electrical read signal each time that the light spot interacts with prewritten patterns inserted between blocks of data.
For controlling the radial position of the optical head, that is to say in order to have the scanning light spot follow a given track element in a set comprising several tens of thousands of such elements, each element is given an identity in the form of a digital address.
This arrangement is used at the end of an access to check the accuracy with which the head is positioned; it may not be sufficient in itself, as it is a lengthy process. Provision is generally made for also checking the advance of the read head by measuring its radial displacement from the start position until addresses near the destination point can be read.
For measuring a displacement it is known to use a member such as a screw to displace the optical head, continuous or stepwise rotation of which indicates the amplitude and sign of displacement. It is also known to associate with a displacement motor a measuring system such as an optical rule which determines the actual displacement of the optical head. These devices have the disadvantage of being slow and/or complex, but they are in any event inadequate in that they do not provide any measure of the displacement of the read light spot relative to the prewritten information. The prewritten information may be eccentric to the rotation axis of the disk with the result that even if the read light spot is fixed in space it periodically traverses several dozen track elements.
To alleviate these disadvantages it is known to use a track crossing counting technique but this presupposes that the track is continuous and that the signals used for counting originate from photosensor means adapted to produce sensed signals in phase quadrature. Counting crossings of a continuous track implies the presence of a prewritten groove in the areas where data is stored and leads to the adoption for other areas of a form of prewritten information that is relatively difficult to employ.
Experience shows that the sampling technique ensures correct track following even though the track is only materialized by a limited number of prewritten patterns grouped together between the blocks of data. Thus a disk is scanned in the same way whether it comprises written data or not, which considerably facilitates the choice of the coatings used to store the data because the writing contrast has no influence on the contrast of the prewritten patterns.
Experience shows that the sampling technique can be extended to counting track crossings even where these occur between two samplings of sensed signals. Also, the track crossing counting method that consists in deducing an address from another address on the basis of the indication provided by a counter can be replaced by address indications read "on the fly" which localize even better the scanning light spot in the memory plane if the track crossings are less frequent relative to the sampling frequency used to acquire samples marking the trajectory. The availability of an accurate knowledge of the position of the scanning light spot relative to the prewritten structure results in particular from the fact that the crossing speed changes relatively slowly enabling a good analysis of the radial access. This is particularly true at the beginning and at the end of an access where the speeds are low.